1. Field of the Invention
The present invention relates to a drive for driving an endless belt and to an image heating apparatus for heating an image on a recording material with the heat of a heater through the endless belt.
2. Description of the Related Art
A heat roller fusing system has widely been adopted as a fusing mechanism for heating and fusing a toner image on a recording material, wherein a heating roller maintained at a specified temperature, and a pressurizing roller having an elastic layer and pressing the heating roller are used to hold and transport, and heat a recording material on which a non-fused toner image is formed.
In recent years, a film heating system proposed in U.S. Pat. Nos. 5,149,941 and 5,043,763 has been recognized as a superb heating system substitute for the heat roller system and put to practical use.
In the film heating system, the use of endless belt-type film is preferred because it obviates the need for a rewind process. However, measures must be taken against displacement of the endless film.
In U.S. Pat. No. 5,027,160, a position of an endless belt in lateral shifting direction is detected, a solenoid is used to displace the position of one end of a follower roller, and then the endless belt is reciprocated within a specified range.
FIGS. 18 to 20 show an example of a heating fusing mechanism that changes the position of one end of a follower roller by turning on or off a solenoid.
FIG. 18 shows a state in which a bearing 135 at the back of a follower roller 124 is pushed down. FIG. 19 shows a state in which the bearing 135 of the follower roller 125 is pushed up by spring 137.
As shown in FIGS. 18 and 19, the bearing 135 of the follower roller 124, which is a pair of a driving roller 125, is supported by a side plate 146 so as to be slidable vertically. One end of the follower roller 124 is supported by the bearing 135 so as to be rotatable, and the other end thereof is supported by a bearing (not shown) formed on a side plate 147 so as to be rotatable.
A locking member 136 mounted on the side plate 146 supports one end of a spring 137 for pushing up the bearing 135. The other end of the spring 137 pushes the bottom of the bearing 135. A spring clutch 138 is made up of an input hub (not shown), a coil spring (not shown) having a control claw (not shown), a control collar 140a formed to hold the control claw, and an output hub 141. When as shown in FIGS. 18 and 19, an engagement claw 140b or 140c engages with a lever claw 144 and the control collar 140a is at a halt, the power of the input hub is not transmitted to the output hub 141. When the lever claw 144 separates from the engagement claw 140b or 140c, the control collar 140a becomes rotatable. The power of the input hub is transmitted to the output hub 141. A torque is always transmitted to the input hub via a gear (not shown) in an arrow-B direction in FIG. 18.
A cam 139 whose radius varies depending on an angle of rotation is fixed to the output hub 141 so as to rotate as part of the output hub 141. When the engagement claw 140b engages with the lever claw 144 as shown in FIG. 18, the lower radius of the cam 139 becomes maximum. When the engagement claw 140c engages with the lever claw 144 as shown in FIG. 19, the lower radius of the cam 139 becomes minimum. The radii of the cam 139 at other positions are determined to provide smooth transition between the maximum and minimum.
When the engagement claw 140b engages with the lever claw 144, the maximum-radius part of the cam 139 pushes down the bearing 139. When the engagement claw 140c engages with the lever claw 144, the bearing 135 is pushed up by the spring 137.
A lever 143 is supported by a spindle 142 implanted in the side plate 146 so as to be rotatable. The lever claw 144 is formed on the other end of the lever 143 which is coupled with an operating beam of a solenoid 145. The solenoid 145 is turned on for a specified period of time according to the signals from sensors 148 and 149 which will be described later.
The sensors 148 and 149 detects that a heat-resistant endless belt 123 has moved behind or ahead of a specified position. The output signals of the sensors 148 and 149 are sent to a microcomputer, whereby the solenoid 145 is turned on or off according to a specified sequence.
In FIGS. 18 and 19, 121 denotes a heat generator and 122 denotes a pressurizing roller. Reference numerals 132 and 134 denote guide plates.
FIG. 20 show the positional relationship between the follower roller 124 and driving roller 125, viewing the follower roller 124 and driving roller 125 from a paper feed section. As described previously, when the minimum-radius part of the cam 139 abuts the bearing 135 as shown in FIG. 19, the spring 137 pushes up the end of the follower roller 124. The follower roller 124 rises rightward as shown in FIG. 20. At this time, the heat-resistant endless belt 123 moves rightward (backward) in FIG. 18. In contrast, as described previously, when the cam 139 pushes down the bearing 135, the heat-resistant endless belt 123 moves leftward (forward).
The aforesaid mechanism requires sensors for detecting the position of an endless belt and a displacement member for displacing a follower roller, thus becoming large in size. Furthermore, since the endless belt is reciprocated forcibly all the time, if thin film is employed, the durability of the thin film deteriorates.